Dr. Gallagher et al1 wrote 22 years ago
that "the role of Mahaim fibers in the genesis of cardiac arrhythmias
in man has been controversial since they were first described " in the
late 30's by Dr. Ivan Mahaim2. The very
early reports were strictly anatomical studies2-6.
This histopathologic quest did not end yet. Mahaim fibers were supposed
to be accessory connections taking off from the His bundle and
fascicles (FV-fasciculoventricular) to the right ventricle or from the
atrioventricular node (NV-nodoventricular fibers) to the right
ventricle. Anderson et al7 proposed 2
varieties of NV fibers, one that arises from the transitional zone and
the other which inserted from the deep, compact nodal portion of the AV
junction. In his pioneering work HJJ Wellens paved the road for
clinical electrophysiological investigation. He was the first to study
a patient with accessory pathway with decremental properties and long
conduction times assuming its relationship with the fibers described
long ago by "Mahaim", as reported in his doctoral thesis8 in 1971. The term nodofascicular (NF) was
applied when the retrograde His bundle potential preceded the
ventricular deflection, while nodoventricular pathway would be
appropriate when the retrograde His bundle deflection followed the
ventricular potential. It took some years to electrophysiologists
realize the conceptual mismatch among the "Mahaim" physiology and
structure described by Mahaim et al. An important observation was done
in 1978 by Becker et al5 who found an
accessory node associated with a bundle
of specialized fibers measuring 1 cm and coursing through the right
ventricle,
mimicking a second AV conduction system located on the lateral
tricuspid
annulus. However, that did not change the mainstream concept of NV
fibers.
During the early 80's many centers started to refer patients with drug
refractory
tachycardias to surgical treatment. According to the current concepts
at
that time targeting the A-V node would be the logic strategy for
curative treatment of patients with NV/NF fibers. Some courageous
electrophysiologists used a new technique consisting of high-energy
catheter ablation of the
A-V node to treat a patient with "Mahaim" fiber, which yielded complete
AV block and persistent preexcitation9. The
turning point came in 1988 at the University Hospital of Western
Ontario, Canada, when Klein, Guiraudon et al10
had decided to extensively freeze the A-V node and upper His bundle
region of a 29 year old man and they soon realized that preexcitation
did not go away. It became clear for them that his accessory pathway
was not linked whatsoever to
the A-V node. The next patient was luckier, and had kept intact his A-V
node, while his "Mahaim fibers" were successfully severed after ice
mapping
produced a consistent zone of reversible block in the accessory pathway
at the right lateral aspect of the tricuspid annulus. Klein's
manuscript was received on August 24, 1987, and published the next year
on JACC. Two months later (October 20, 1987) Circulation received a
manuscript from Tchou
P et al11 entitled "Atriofascicular
connection or a nodoventricular fiber? Electrophysiologic elucidation
of the pathway and associated reentrant circuit". From a single case
report we were taught how simple it is to make sure that such pathways
arise from the atrium. In recent years catheter ablation techniques
have shed more light on the subject. Discrete "Mahaim" potentials that
are considered surrogates of pseudo-Mahaim
tissue depolarization, are used as an effective target for ablation12,13. A number of pharmacologic14 and histologic data5,6,15,16,
electrophysiologic
maneuvers and observations during radiofrequency catheter ablation like
heat induced "Mahaim" automaticity19,20 are regarded as evidences of either an ectopic
A-V
node or remnants of the specialized A-V ring tissue. The NV/NF fibers
are
now considered a rare item but there are some convincing reports21 of narrow and regular QRS tachycardias with
ventriculoatrial dissociation. The last variety which is known as
fasciculoventricular pathway22 seems to
play no role in clinical tachycardias but as long as it is very often
associated with bypass tracts they should
be correctly recognized and not targeted for ablation, avoiding
unnecessary damage to the A-V node-His bundle conduction system.

RecognitionElectrocardiographic
Features:Baseline electrocardiogram
of patients with atriofascicular or atrioventricular pathways
(pseudo-Mahaim) (Figure 1) are characterized by minimal or no
preexcitation. Sometimes the only clue is absence of septal Q wave in
leads V5 or V623. Some patients show a
typical LBBB with normal PR
interval. A preexcited ECG is more likely to occur in an
atrioventricular decremental pathway24.
Precordial transition (R/S >1) usually occurs at V4 or V5 (sometimes
V6). Latent preexcitation has recently been reported, in patients with
spontaneous LBBB-like antidromic tachycardia, without preexcitation at
rest and during atrial pacing25. A high
degree of day-to-day variability as far as preexcitation is concern,
the "concertina" effect is observed in many patients. Anterograde
conduction over atriofascicular fibers yields a typical LBBB pattern
with variable axis, superior frontal plane axis being the
most commom one (ranging from -25°
to -60°),
but it is of no help in differentiating it from the atrioventricular
pathways. QRS complex is usually larger with anterograde conduction
over an atrioventricular pathway, with a slurred QRS onset26 due to distal muscular insertion, which can be
better appreciated in the r wave of V2 to V4 (>40 msec in
atrioventricular pathways)23.

Electrocardiogram of fasciculoventricular pathway is characterized by
normal frontal plane axis like an anteroseptal accessory pathway (0°
to +75°)27 with a subtle preexcitation and normal PR
interval. It is also commom to see a short PR interval due to
associated enhanced A-V nodal conduction. Atrial pacing do not change
the degree of preexcitation. Junctional beats are preexcited and
intravenous adenosine yields blocked P waves. Precordial transition
(R/S >1) usually occurs at V2 (Figure 2).

Figure 2 : Three cases
of fasciculoventricular pathways: PR intervals are 0,08 sec, 0,09 sec
and 0,10 sec respectively. QRS is wider in the first ECG and <0.11
sec in the others 2 cases, and frontal plane axis are +30°,
+75° and +45°. QRS transition occurs in V3, V2
and V2, respectively.

Electrophysiologic
Characteristics:The major findings are
the slow conducting and decremental properties that can be assessed
during right atrial pacing: AH interval lengthens, HV interval shortens
and QRS widens until a steady value is achieved. Faster atrial
stimulation does not
increase preexcitation, but can prolong AV conduction time until block
occurs
(Figure 3). Decremental conduction is usually defined as rate
dependent
prolongation of conduction time in more than 30 msec through the
accessory
pathway (AP) as measured in the electrograms close to the AP insertion.
Atrial extrastimulus testing produce likewise results increasing
progressively
AV interval and preexcitation up to a steady value. During atrial
pacing,
achievement of maximal preexcitation is associated with retrograde
conduction
over the right bundle, His bundle and A-V node and cessation of pacing
is
usually followed by antidromic tachycardia (Figure 4). The next
step
is to assess the role of the accessory pathway in the tachycardia
circuit:
active or bystander. It can be done delivering single late lateral
right
atrial extrastimuli during preexcited tachycardia, timed not to affect
the
His region and coronary sinus atrial electrogram at the ostium11. Advancement of QRS complex and atrial
activation establishes the diagnosis of an extranodal accessory pathway
(proximal atrial insertion) as well its involvement in the tachycardia
circuit. If advancement
of QRS activation occurs without changing of atrial activation, the
presence
of an extranodal AP is certain but its participation on the circuit is
not.
McClelland et al could successfully advance QRS activation with late
right
atrial extrastimuli in 22 of 23 patients with atriofascicular
tachycardia13. Another less elegant
maneuver proving AP participation
in SVT is by producing catheter-induced RBBB. Assuming an antidromic
tachycardia
incorporating an atriofascicular (or atrioventricular) pathway
retrograde
conduction occurs via right bundle-His bundle and A-V node axis. RBBB
lengthens
the circuit path, tachycardia cycle length due to an increase in
ventriculoatrial
time. Preexcited A-V nodal reentry as well as antidromic tachycardia
with
retrograde conduction through another AP would not be affected. The
proximal
atrial insertion can also be localized with the recording of an
accessory
pathway potential, because it is usually recorded in the
postero-lateral
or antero-lateral aspect of tricuspid annulus, away from the A-V node.
In
the study of Grogin et al28 clues as to
the presence of a nodoventricular fiber were the inability of a
premature atrial stimulus to advance the ventricle and the presence of
dual A-V nodal pathways. Scheinman et al29
were not agreeable with that
statement and finding an "M" potential assumes diagnostic importance in
this setting, because the "M" potential accurately localizes the
anatomic
site of the pathway.

Figure 3:
Atrioventricular pathway: high right atrial (HRA) pacing at 450 msec
causes Wenckebach block on the accessory pathway. Progressive
prolongation of AV interval (80-120-150-block) is due to prolongation
of A-AP potential (40-80-115-block). Preexcitation degree increases
from the first to the second QRS complex and remains
constant in the third QRS despite further prolongation of the A-APP
interval.

Figure 4: Antidromic
tachycardia. Late (S) atrial extrastimuli delivered from the lateral
high right atrium without disturbing AA timing at the His bundle
recording advances QRS complex by 20 msec and His deflection by 30
msec, proving that the pathway is
extra nodal and participates on the circuit.

Ventricular stimulation usually discloses ventriculoatrial conduction
from the A-V node. The vast majority of atriofascicular and
atrioventricular
pathways have only anterograde conduction30, 31. Adenosine injection during sinus rhythm can
yield complete AV block or
sometimes increase preexcitation32.
Verapamil has a more proeminent effect over the A-V node, and can be of
help in
exposing preexcitation. During antidromic tachycardia adenosine causes
prolongation of conduction over the pathway and eventual block,
terminating
tachycardia.
There are plenty of electrophysiologic and anatomic data supporting the
concept that at least atriofascicular pathways are accessory AV nodes.
We have seen a patient with an electrophysiologic profile suggestive of
the presence of an atriofascicular pathway without conduction through
the
AV node. This patient had unexplained syncope with a baseline ECG
showing
LBBB-like pattern with normal PR interval (Figure 5a). Atrial
pacing
disclosed a decremental AP and 1:1 conduction up to 280 msec with an
"M"
potential in right posteroseptal region. There was no VA conduction.
Adenosine
yielded transient complete AV block. We decided to ablate the
Mahaim-like
pathway and as soon as radiofrequency current was delivered, the
patient
had developed complete AV block (Figure 5b). Ablation was
immediately
discontinued and patient resumed preexcitation. The transient escape
rhythm
after ablation had a normal HV interval but no conduction occurred over
the AV node. This scenario is consistent with an ectopic accessory AV
node
without conduction from the "normal" AV node.

Figure 5b: During
atrial pacing
a brief pulse (1 sec) of radiofrequency current delivered at a site
with
"M" potential yield a transient complete AV block followed by an escape
rhythm without preexcitation (30-35 b/min). Preexcitation resumed in 14
seconds

Associated conditions: Mahaim fibers (AF/ AV) very often occurs in the
setting of Ebstein's disease (10 to 40%) and associated accessory
pathways
are a commom finding (up to 30%). Fasciculoventricular pathways also
occurs
very often in association with accessory pathways. I have reviewed the
cases reported since 19811, 22,23,33,34
together
with 3 cases from our own laboratory, and I found 6 of 15 patients with
associated AP's (40%). Dual AV nodal pathways with AV nodal reentrant
tachycardia is much higher of that expected by chance alone (Table 1).

Table 1: Data from
those series suggest that authors using the "M" potential as the
mapping technique for ablation are more successful with less RF pulses
and lower recurrence rate.

Treatment

Mapping and Catheter
Ablation:Some particular
features are unique to Mahaim fibers: Mapping of the atrial insertion
by ventricular stimulation is usually not possible because those
decremental pathways do not conduct retrogradely. Atrioventricular
connections can be located by mapping the site of earliest ventricular
activation on the annulus, as
with other anterogradely conducting accessory AV pathways. On the
contrary, atriofascicular pathways or even the long atrioventricular
pathways with distal (nonannular) insertion cannot be mapped in this
way. To worse matters these decremental pathways are unusually
sensitive to mechanical trauma. Inadvertent knocking of the ablation
catheter against the annulus can result in transient abolition of
conduction through the pathway from minutes to hours34,35. The following strategies have been used to
overcome those problems:

1.
Searching for the "M" (Mahaim) potential (Figure 6) along the
tricuspid annulus is the most commonly used technique. The ablation
catheter should be carefully moved along the annulus avoiding bumps on
the tissue. We routinely use a long sheath like DAIG®
SR2 or
SR3, which improves stability. The potential may be as large as the His
bundle potential or small, narrow with low amplitude. Catheter ablation
at a site with "M" potential is likely to be successful (Table 1).
We20 and other authors19,29 have recorded automatic rhythms ("Mahaim"
automatic tachycardia-MAT) brought about during radiofrequency current
delivery. It is probably due to heat-related automaticity of nodal-like
tissue in a similar fashion to junctional rhythm that arises during
slow A-V nodal pathway ablation. It seems to represent a hallmark for
successful ablation particularly of atriofascicular pathways. MAT in
most cases is short-lived (Figure 7), but ocasionally it lasts
longer. We have had an early out of hospital recurrence when it was not
our policy to completely eliminate such rhythm. In a second procedure
we decided
to ablated until complete elimination of automatic activity (Figure
8). We have seen MAT during radiofrequency catheter ablation of
atriofascicular pathway but not with atrioventricular pathways.

2. Activation
mapping of
the earliest local ventricular potential is feasible in short
atrioventricular pathways like in fast conducting AV accessory
pathways. Atrioventricular decremental pathways with a long course
often shows extensive arborization over a wide area of ventricular
muscle23. Targeting distal branches is a
time consuming task. It is possible to ablate some of them, as assessed
by changes in preexcitation pattern, but a complete elimination is very
unlikely. Some patients with atriofascicular pathway who underwent
ablation at the distal insertion had developed a proarrhythmic13,35 response with
facilitation
of antidromic tachycardia occurrence due to slow conduction induced by
radiofrequency ablation.

3. Shortest
stimulus-QRS interval as assessed by atrial stimulation at a constant
pacing rate along the atrial aspect of the annulus was the gold
standard mapping method before mapping of "M" potential had been
reported. Stimulation sites remote from the atrial insertion of the
accessory pathway result in long stimulus-QRS interval due to the
amount of interposed atrial tissue. We do not use this method because
it is time consuming and very inaccurate because it is
difficult to stimulate from many sites at the same distance from the
annulus
and stimulating atrial tissue requires good contact with the tip, which
is not always possible.

4. Extrastimulus
mapping during antidromic tachycardia. Finding an atrial site where the
longest coupled premature extrastimulus causes resetting, or assessing
the amount of advancement of the QRS following application of a fixed
atrial extraestimulus coupling interval. Similar to the previous
technique it looks for a site in the atrial annulus with the least
interposing tissue separating it from the accessory pathway proximal
insertion. Likewise shortest stimulus-QRS technique is an inaccurate
and tedious method.

5. Some authors36,37 reported the judicious
use of
mechanical trauma in a controlled way and transient AP conduction block
to
find the AP insertion. The rationale of this method is based on the
observation that transient conduction block following bumps of the
catheter on the atrial aspect of the annulus is a frequent phenomena in
decremental anterograde pathways. Gentle pressure of the tip of a
steerable mapping catheter during antidromic tachycardia or atrial
pacing would lead to transient block of AP conduction locating the site
for ablation. The fact that those AP are more prone to mechanical
trauma suggests that they may be thinner and
have a more superficial location than others. The main pitfalls are:
when
conduction block occurs the catheter is on the move and may be away
from
the AP site, and its relocation will be dependent on resuming of AP
conduction;
mechanical block can last hours and a second procedure would be needed.
I do not favor the use of this method and I'd rather avoid mechanical
trauma.

6. Electroanatomic
mapping (noncontact mapping)38 can be
helpful, while not widely available, in those cases where accessory
pathway potential cannot be found and when mechanical trauma precludes
adequate mapping. This
technique allows the operator to "tag" the exact location of the tip of
the catheter and in case of transient conduction block the catheter can
be manipulated back to the tagged location for ablation.

Figure 6: Three examples of "M"
potentials (TA- tricuspid annulus electrograms): from left to right:
first two cases with His-like potentials and the third with narrow and
low amplitude potential. Ablation was successful in each of those sites.

Figure 8: Radiofrequency ablation of
recurrent atriofascicular pathway conduction: during ablation salvos of
automatic and irregular rhythms with the same QRS morphology as the
preexcited one. The salvos persisted even after abolition of
preexcitation (*) and fade out until complete disappearance.

Radiofrequency current should be applied during atrial pacing to
enhance preexcitation and making it easier to assess conduction block
at the AP. Stability is improved during atrial pacing as compared with
ablation during antidromic tachycardia when catheter is likely to move
with tachycardia termination. MAT is a common and expected event and
can also cause catheter displacement. Before "M" potential mapping
technique became the gold standard mapping technique some authors
favored targeting the ventricular insertion to avoid MAT and maintain a
better catheter stability39.
Catheter ablation have been very successful particularly when ablating
at a site with "M" potential or assessing earliest delta-V interval in
atrioventricular decremental pathways (Table 1).

Acknowledgment

I express my gratitude to Luiz Marcio Gerken MD, for his
invaluable help and support in the performance of the studies that form
the basis of this review.